Ever since Owen and Medawar, transplanters have dreamed of harnessing the power of neonatally-acquired tolerance. A recent collaborative retrospective study of 198 sibling LRD renal transplant (RTX) recipients at 9 transplant centers in the US and the Netherlands has revealed a powerful beneficial influence of neonatal exposure to HLA non-inherited maternal antigens (NIMA) on survival of 1 HLA haplotype-matched living donor renal allografts. The 10 year graft survival of NIMA-HLA mismatched haploidentical sibling RTX was equivalent to that of HLA-identical siblings. Paradoxically, the beneficial effect of NIMA versus NIPA mismatch was seen despite a significant (p less than .05) tendency toward earlier and greater cumulative incidence of first rejection episodes in the former. We hypothesize that the beneficial NIMA effect involves three separate components: 1) tolerization of allospecific B cells by soluble maternal HLA-NIMA present in placental blood and in milk, 2) tolerization of "indirect pathway" T helper cells by soluble maternal HLA-NIMA reprocessed and presented as peptides by immature APC of the neonate, and 3) immunization of "direct pathway" allospecific T cells and a subset of "indirect pathway" T cells by cell membrane-bound and soluble NIMA-HLA presented by mature APC of the mother. To test the role of soluble vs. cell-bound MHC class I antigens in the phenomenon of non-inherited maternal antigen (NIMA)-induced tolerance we will: 1) analyse the biochemical forms of NIMA HLA proteins that contact the developing immune system, and compare the direct and indirect pathway T cell responses to these forms in normal adults; 2) determine the in vivo and in vitro correlates of the difference in NIMA vs. NIPA-mismatched sibling renal allograft survival; and 3) analyze the mechanism of the NIMA transplant tolerance effect in a mouse vascularized allograft model. Our goal is to establish the basis for a rational approach to HLA-matching and selective NIMA mismatching to achieve the goal of clinical kidney transplantation tolerance.